Tony Blair has declared that airpower alone is not enough to defeat IS. In a 6,500-word essay, the former Prime Minister stated that while no desire existed for ground engagement in the region, airpower alone would not be enough to defeat the group.

"We have to fight groups like ISIS," he said. "There can be an abundance of diplomacy, all necessary relief of humanitarian suffering, every conceivable statement of condemnation which we can muster.

"But unless they're accompanied by physical combat, we will mitigate the problem but not overcome it.

"Airpower is a major component of this to be sure, especially with the new weapons available to us. But - and this is the hard truth - airpower alone will not suffice.

"If possible, others closer to the field of battle, with a more immediate interest, can be given the weapons and the training to carry the fight.

"I accept fully there is no appetite for ground engagement in the West. But we should not rule it out in the future if it is absolutely necessary."

The militant Islamist group has made rapid territorial gains across the region and released graphic videos depicting the beheading of two US journalists and British aid worker David Haines.

Mr Blair warned that any solution to the threat posed by the extremist group would involve casualties. 

"Because the enemy we're fighting is fanatical, because they are prepared both to kill and to die, there is no solution that doesn't involve force applied with a willingness to take casualties in carrying the fight through to the end," he said.

The US and France have already launched airstrikes against IS targets, and the UK has not ruled out joining the bombing campaign.

He said the lessons learned from the wars in Afghanistan and Iraq had improved Western forces' "capacity and capability" to respond to the threat of IS and similar groups.

"To those who say that after the campaigns in Afghanistan and Iraq we have no stomach for such a commitment, I would reply the difficulties we encountered there are in part intrinsic to the nature of the battle being waged," he said.